The present invention relates to radial flow adsorption units for separating a gaseous component from a gas mixture, and particularly for purifying air prior to cryogenic distillation. The invention is primarily concerned with units configured for U-flow operation.
It is standard practice in the cryogenic air separation industry to use radial flow adsorption units for removing contaminants such as water, carbon dioxide, trace hydrocarbons and NOx from the air feed to the cryogenic air separation plant to avoid issues with plant operation and safety.
A radial flow adsorption unit for air purification is typically a vessel having an outer tubular side wall that is closed at each end with a respective end wall, containing an elongated annular bed of at least one adsorbent material located co-axially within the interior of vessel. There is usually an annular space defined by the inside surface of the outer wall and the outside surface of the annular bed. The annular bed defines a central channel. There is a gas inlet that feeds the gas to be processed to the annular space, and a gas outlet that removes treated gas from the central channel.
In operation, air is usually fed to the annular space surrounding the annular bed. The air passes through the adsorbent material(s) within the annular bed into the central channel. The adsorbent material(s) selectively adsorb at least one contaminant from the air thereby producing purified air which is removed from the vessel.
Less commonly, air to be purified can be fed to the central channel defined by the annular bed and purified air removed from the annular space surrounding the annular bed. In such arrangements, the air passes through the annular bed in the opposite radial direction, i.e. flowing from the central channel to the annular space.
The adsorbent beds are typically regenerated by passing a regeneration gas through the adsorbent bed in the opposite direction to the direction of the air when the unit is in operation, i.e. “on feed” or “on line”.
In general, radial flow adsorption units may be operated using a temperature swing adsorption (“TSA”) process, a pressure swing adsorption (“PSA”) process, a vacuum swing adsorption (VSA) process or a vacuum pressure swing adsorption (VPSA) process, or using modifications of such processes as known in the art. However, units involved in air purification are typically operated using a TSA process, and units involved in the bulk separation of air are typically operated using a PSA or VPSA process. An example of an air purification process generating the feed to a cryogenic air separation unit (“ASU”) that involves radial TSA technology is disclosed in U.S. Pat. No. 5,855,685A.
Radial flow adsorption units may be configured in a “U-flow” or “Z-flow” arrangement. In a U-flow arrangement, the paths of the gas on either side of the annular bed are in opposite directions. “U-flow” is also referred to as “π-flow” in the literature. In a Z-flow arrangement, the paths of the gas on either side of the annular bed are in the same direction. In front end air purification in the air separation industry, the primary focus is on the use of radial flow adsorption units configured in a Z-flow arrangement. U.S. Pat. No. 4,541,851A, U.S. Pat. No. 5,827,485A, U.S. Pat. No. 8,313,561B, US2010/0058804A and US2011/0206573A each disclose radial flow adsorption units for air purification in which the units are configured in a Z-flow arrangement. A feature of such units is that the gas inlets and gas outlets are usually at opposite ends of the units.
A feature of a typical Z-flow configuration in a radial flow adsorption unit is unequal pressure drop along the length of the adsorbent bed which leads to non-uniform flow distribution. Attempts to overcome the unequal pressure drop include the use of internal components in the central channel, such as conical baffles. However, incorporating such internal components typically complicates the design of the unit, thereby increasing the overall capital costs, and increases the overall pressure drop through the vessel, thereby increasing the overall operating costs.
The U-flow configuration offers an attractive alternative for radial flow adsorption units since the pressure drop is typically equal along the length of the adsorbent bed resulting in more uniform flow distribution without the need for additional internal components, thereby potentially reducing overall capital and operating costs.
U.S. Pat. No. 5,814,129A discloses radial flow adsorption unit for the pre-purification of air. The unit has a gas inlet at the bottom of the vessel and a gas outlet at the top of the vessel but has been configured for “U-flow” using a cylindrical baffle provided either in the annular space between the side wall of the vessel and the adsorbent bed (see FIG. 1), or in the central channel defined by the annular adsorbent bed (see FIG. 3). The baffle forces the gas to flow in the annular space towards the top of the vessel from the inlet before passing through the bed (see FIG. 1), or towards the bottom of vessel after passing through the bed and before being removed via the outlet (see FIG. 3). A similar arrangement is disclosed in U.S. Pat. No. 5,759,242A (see FIG. 1).
While the radial flow adsorption units disclosed in U.S. Pat. No. 5,814,129A represent an improvement over typical units configured for Z-flow because of the improved flow distribution through the adsorbent bed, the design of the unit is still more complicated than ideal and the presence of the cylindrical baffle increases overall pressure drop through the vessel. Therefore, there is still a need for new designs of radial flow adsorption units.
In terms of uniform flow distribution, the ideal arrangement for a radial flow adsorption unit is theoretically a U-flow configuration with a co-axial gas inlet and gas outlet at one end of the unit since this arrangement potentially provides the most uniform flow distribution. Examples of references disclosing such arrangements include U.S. Pat. No. 5,759,242A (see FIG. 4). The units are intended primarily for PSA or VPSA operation in an oxygen cycle but the reference mentions that the units can be modified for use in PSA pre-purification of air. However, the units are complicated mechanically which increases the capital cost.
Further radial flow adsorption units for air purification which are configured in a “U-flow” arrangement are disclosed in U.S. Pat. No. 8,313,561B (see FIG. 2(e)). This reference mentions that the unit can be configured so that the air inlet and outlet are both either in the top wall or the bottom wall of the vessel, and that the air to be purified can be fed either to the central channel defined by the annular bed, or to the annular space between the annular bed and the outside wall of the vessel.
In addition to radial flow adsorption units configured in a “Z-flow” arrangement, U.S. Pat. No. 4,541,851A also discloses (see FIG. 4) such a unit configured in a “U-flow” arrangement for air purification. The unit has an air inlet in the bottom wall of the unit that feeds the central channel defined by the annular bed of adsorbent material. The air passes through the adsorbent bed to the annular space between the bed and the side wall of the unit. The purified air is removed from unit using a gas outlet also provided in the bottom wall of the unit.
An example of a radial flow adsorption vessel configured in a “U-flow” arrangement for the bulk separation of air in which the gas inlet and gas outlet are located separately in the bottom wall of the vessel is disclosed in U.S. Pat. No. 5,232,479A (see FIG. 1).
In general, radial flow adsorption units tend to be large, particularly for certain applications where a large amount of gas needs to be processed. An example of such an application is front-end air purification for a cryogenic ASU. Such units may have an overall height/length of up to 25 m and the associated pipework may have a diameter of up to 72 inches (1.8 m) for larger plants, e.g. 56 inches (1.4 m) for the gas inlet and 42 inches (1 m) for the gas outlet.
Units are typically orientated vertically to reduce the size of their footprint. Height is a particular issue for units configured for Z-flow since such units tend to have their inlets and outlets at opposite ends of the units. This means that there will be piping exiting the ‘top’ of the vessel and running down to ground. Piping at low level is easier and less expensive to construct, support and maintain than piping at high levels. Units configured for U-flow tend to have their inlets and outlets in the same end wall which mitigates somewhat the issues due to height. However, the size of the pipework means that the head having the inlet and the outlet can be very congested which limits the options for the layout of the pipework.
Radial flow adsorption units are typically capable of being pressurized to at least 5 bar and possibly up to 40 bar. Thus, it is necessary to reinforce the head around each gas inlet and gas outlet by thickening the wall. Where the gas inlet and gas outlet are in the same head, the reinforcements can overlap resulting in an even thicker end wall.
The walls of a radial flow adsorption unit are typically made from carbon steel. If the thickness of a wall made from this material exceeds about 38 mm, then the unit must undergo post-weld heat treatment in which the entire shell of the unit or just the thicker component parts are heated in a furnace to a high temperature, e.g. from about 550 to about 600° C., for a period of time (depending on the thickness), e.g. 0.5 hours, as defined by the relevant pressure vessel fabrication code. In addition, the heating and cooling rates must be carefully controlled, together with the atmosphere inside the furnace. The size of the units is such that the furnace often has to be built around the unit. Post weld heat treatment is therefore an expensive process which is desirable to avoid if possible.
The Inventors also note that the end wall, or “head”, of a radial flow adsorption unit tends to have a larger diameter if two or more gas inlet/outlet nozzles are located there. Since radial flow adsorption units are usually fabricated in a factory and then transported to site at least in part by road using a flatbed lorry, larger diameter units tend to be more difficult to transport, e.g. on narrow roads with low bridges. Alternatively, if the end wall is made of optimal diameter, then some compromise may have to be made regarding the inlet and outlet nozzle sizes (they may have to be smaller than desired, leading to higher pressure drop).